Add stopPropagation option on dropdown

[vbornand]

Fix the issue #451

[vbornand]

This PR add this option only to the custom attribute and not on the element.

[vbornand]

I updated the PR, now it add the option to the attribute and to the element.

[Thanood]

Thanks a lot! 👍
The element is a placeholder, btw. I've tried that in the beginning and went with the attribute instead. Maybe we should remove that.

One thing: could you use a commit message which corresponds to our commit message convention? The format is readable by conventional-changelog and will be used to generate release notes.

For this case it would be: feat(md-dropdown): add stopPropagation option.
No need to change the commits now, I can adapt the changelog manually. Just for next time maybe. 😃

[vbornand]

I will open again a pull request with a single commit and clean message. Don't spend your time to correct this one.